Growth factors are substances, such as peptides, which affect the growth and differentiation of defined populations of cells in vivo or in vitro.
Bone formation occurs during development of long bones (endochondral bone formation) and flat bones (intramembranous bone formation). Further, bone formation occurs during bone remodeling which occurs continuously in adult life in order to preserve the integrity of the skeleton. Finally, bone formation occurs during bone repair, such as when bone wounds occur in a fracture or surgical situation, for example. While separate bone formation mechanisms are thought to be involved in the embryological development of long and flat bones and repair is thought to involve intramembranous bone formation.
Bone formation by either mechanism involves the activity of osteoblasts, which are regulated by growth factors. Osteoblasts are derived from a pool of marrow stromal cells (also known as mesenchymal stem cells; MSC). These cells are present in a variety of tissues and are prevalent in bone marrow stroma. MSC are pluripotent and can differentiate into a variety of cell types including osteoblasts, chondrocytes, fibroblasts, myocytes, and adipocytes. Growth factors are thought to impact osteogenic cell proliferation, differentiation and osteoblast mineralization, each of which impacts bone formation.
Autogenous bone has been used, such to repair bone in patients with craniosynostosis and cleft grafting, for example. Craniosynostosis (CS), the premature closure of cranial sutures, affects 1 in 3,000 infants and therefore is one of the most common human congenital craniofacial deformities. Premature suture closure results in cranial dimorphism, which can need surgical correction. Premature suture closure in human CS can occur by two possibly distinct processes: calvarial overgrowth and bony fusion. Recently, fibroblast growth factor 2 (FGF2) and fibroblast growth factor receptor 1 (FGFR1) have been implicated in premature cranial suture fusion via CBFA1-mediated pathways (8). Missense mutation of CBFA1 is linked to cleidocranial dysplasia, manifested as delayed suture closure.
Autologous bone grafting procedures have been performed utilizing autogenous bone, such as from the iliac crest or calvaria. These donor sites are not without associated morbidity including pain, gait disturbance, thigh paresthesia for iliac crest donor sites, and infection, neurologic deficits, and hematomas for calvarial grafts. Further, donor sites can have limited volume and can contribute to increased surgical time and hospital stay.
Alloplastic grafting materials have also been utilized, and growth factors have been tested in animal models. For example, bFGF has shown potential for use in bone regeneration and repair. Another family of osteogenic growth factors have been described as bone morphogenic protein (BMP). Specifically, BMP-2 recombinant protein has been demonstrated to regenerate mandibular continuity defects and cleft palate defects with results equal to or better than autogenous particulate bone and marrow. BMPs and other osteogenic factors have been studied for use in clinical applications. However, the cost of using minimally effective dosages of BMP has been a limiting factor in clinical use.
Spinal fusion is a surgical technique in which one more of the vertebrae of the spine are united together so that motion no longer occurs between them. Indications include: treatment of a fractured (broken) vertebra, correction of deformity, elimination of pain from motion, treatment of instability, and treatment of some cervical disc herniations. The surgery can involve placement of a bone graft between the vertebrae to obtain a solid union between the vertebrae. The procedure also can involve supplemental treatments including the placement of plates, screws, cages, and recently bone morphogenic protein 2 and 7 to assist in stabilizing and healing the bone graft. Autogenous bone grafting has been the clinically preferred method, and yet has about a 30-50% failure rate. Autogenous bone grafting is a separate surgery and also carries significant morbidity.
Cartilage is a type of dense connective tissue. It is composed of chondrocytes which are dispersed in a firm gel-like matrix. Cartilage is avascular (contains no blood vessels) and nutrients are diffused through the matrix. Cartilage is found in the joints, the rib cage, the ear, the nose, in the throat and between intervertebral disks. There are three main types of cartilage: hyaline (e.g., costal cartilages, the cartilages of the nose, trachea, and bronchi, and the articular cartilages of joints), elastic (e.g., external ear, external auditory meatus, part of the Eustachian tube, epiglottis, and in some of the laryngeal cartilages) and fibrocartilage [e.g. meniscus (e.g., wrist triangular fibrocartilage complex, knee meniscus), intervertebral discs, temporomandibular joint disc, the pubic symphysis, and in some tendons and ligaments at their attachment to bones. One of the main purposes of cartilage is to provide a framework upon which bone deposition could begin (i.e., during endochondral ossification). Another important purpose of cartilage is to provide smooth surfaces for the movement of articulating bones. For example, articular cartilage, most notably that which is found in the knee joint, is generally characterized by very low friction, high wear resistance, and poor regenerative qualities. It is responsible for much of the compressive resistance and load bearing qualities of the knee joint and, without it, walking is painful to impossible. Yet another important purpose of cartilage is to provide, firm, yet flexible support (e.g., nasal cartilage, spinal discs, tracheal cartilage, knee meniscus, bronchial cartilage). For instance, cartilage such as the meniscus plays a crucial role in joint stability, lubrication, and force transmission. Under a weight bearing load, the meniscus maintains the balanced position of the femur on the tibia and distributes the compressive forces by increasing the surface contact area, thereby decreasing the average stress two to three times. Additionally, the menisci interact with the joint fluid to produce a coefficient of friction that is five times as slick as ice on ice. In another example, the intervertebral disc has several important functions, including functioning as a spacer, as a shock absorber, and as a motion unit. The gelatinous central portion of the disc is called the nucleus pulposus. It is composed of 80-90% water. The solid portion of the nucleus is Type II collagen and non-aggregated proteoglycans. The outer ligamentous ring around the nucleus pulposus is called the annulus fibrosus, which hydraulically seals the nucleus, and allows intradiscal pressures to rise as the disc is loaded. The annulus has overlapping radial bands, not unlike the plies of a radial tire, and this allows torsional stresses to be distributed through the annulus under normal loading without rupture. The disc functions as a hydraulic cylinder. The annulus interacts with the nucleus. As the nucleus is pressurized, the annular fibers serve a containment function to prevent the nucleus from bulging or herniating.
Cartilage can be damaged by wear, injury or diseases. As we age, the water and protein content of the body's cartilage changes. This change results in weaker, more fragile and thin cartilage. Osteoarthritis is a common condition of cartilage failure that can lead to limited range of motion, bone damage and invariably, pain. Due to a combination of acute stress and chronic fatigue, osteoarthritis directly manifests itself in a wearing away of the articulating surface and, in extreme cases, bone can be exposed in the joint. In another example, loss of the protective stabilizing meniscus leads to increased joint laxity or abnormal motions that lead to joint instability. The excessive motion and narrowed contact area promotes early arthritic changes. At the cellular level, there is initially a loss of cells from the superficial layer of the articular cartilage followed by cartilage splitting, subsequent thinning and erosion occurs, and finally protrusion of the underlying raw bone. The earliest arthritic changes have been noted three weeks after loss of the entire meniscus. In yet another example, because both the discs and the joints that stack the vertebrae (facet joints) are partly composed of cartilage, these areas are subject to wear and tear over time (degenerative changes). As the inner nucleus dehydrates, the disc space narrows, and redundant annular ligaments bulge. With progressive nuclear dehydration, the annular fibers can crack and tear. Loss of normal soft tissue tension may allow the spinal segment to sublux (e.g. partial dislocation of the joint), leading to osteophyte formation (bone spurs), foraminal narrowing, mechanical instability, and pain. If the annular fibers stretch or rupture, allowing the pressurized nuclear material to bulge or herniate and compress neural tissues, pain and weakness may result. This is the condition called a pinched nerve, slipped disc, or herniated disc. Radiculopathy refers to nerve irritation caused by damage to the disc between the vertebrae. Mechanical dysfunction may also cause disc degeneration and pain (e.g. degenerative disc disease). For example, the disc may be damaged as the result of some trauma that overloads the capacity of the disc to withstand increased forces passing through it, and inner or outer portions of the annular fibers may tear. These torn fibers may be the focus for inflammatory response when they are subjected to increased stress, and may cause pain directly, or through the compensatory protective spasm of the deep paraspinal muscles.
NELL peptide has been documented to stimulate bone formation and cartilage formation. Existing methods and systems of producing NELL peptides have limited success with problems. A common problem of existing NELL peptide expression systems (e.g., a mammalian cell such as a CHO cell expression system) is low productivity. Low productivity is generally found to be caused by, for example, an inappropriate construct of the nucleic acid sequence expressing a NELL peptide and/or other additional sequences, e.g., a secretory signal peptide sequence, in a particular expression system, e.g., a CHO cell.
Therefore there is a continuous need for a method and system for producing NELL peptides.